1. Field of the Invention
The present invention relates to optical telecommunication systems and, in particular, to an apparatus and method of manufacturing optical devices employed in such telecommunication systems.
2. Technical Background
Up to three port filtering and isolating packages are widely used in local and long distance optical telecommunication networks. These networks comprise various spectral shaping and isolating optical assemblies as parts of dense wavelength division multiplexing (DWDM) systems. The necessity to design reliable optical devices for such systems, which are subject to various thermal and mechanical loads during their 20 to 25 year lifetime, is of significant importance. A typical example of such optical devices is an optical filter assembly. A typical optical filter assembly comprises two (input and reflective) optical glass fibers inserted into a dual-capillary ferrule to produce a fiber ferrule sub-assembly, a GRIN lens, and a filter. The optical components of the filter assembly are embedded into an insulating glass tube, which in turn is mechanically protected by a metal housing. In a typical 3-port package, the above dual-fiber filtering assembly is combined with an output collimating assembly leading to a single optical fiber. These filter assemblies typically exhibit insertion losses higher than desired, resulting in degraded overall performance of the communications system or module. The problem is particularly acute during exposure to ambient operating conditions where temperature is variable.
Typical input glass ferrules employ one of two designs. A single capillary suitable for containing multiple glass fibers or separate circular capillaries for each fiber have been used, each with relatively short (0.7-1.2 mm) fiber-receiving conical lead-in ends. With such input ferrules, the optical fiber is subjected to an S-bending over the short conical end portion which typically exceeds 50% of the fiber diameter (for a fiber having a 125 xcexcm diameter) on a span of about 6 to 10 diameters in length. This excessive micro-bending increases the insertion losses. Although the multi-capillary design reduces the lateral deflection of fiber interconnects compared to the elliptical single-capillary design, the short length of the cone end of such ferrules cannot reduce the micro-bending of the fiber and its inherent insertion loss. Fiber ferrule subassemblies employing such ferrules are manufactured by inserting the optical fibers stripped of their polymer coating into the respective ferrule capillaries; epoxy bonding the fibers into the ferrule capillaries, including the conical end portions; grinding and polishing an angled facet on the fiber ferrule; and depositing on the polished surface an anti-reflection (AR) coating. Once finished, the fiber ferrule is aligned and assembled with the collimating GRIN lens and then embedded into the insulating glass tube, which, in turn, is protected by a metal housing.
There are two different technical solutions used in the design of bonds securing the components of an optical assembly. A low compliance bond between thermally well-matched glass fibers and the glass ferrule is an approach commonly used by some manufacturers. The adhesives used are heat-curable epoxies with high Young""s modulus (E greater than 100,000 psi) and moderate to high thermal expansion coefficients (xcex1=40-60 10xe2x88x926xc2x0 C.xe2x88x921). A typical example would be 353 ND EPO-TEK epoxy adhesive. In addition, the bond thickness used is very small.
Silicon adhesives are used to bond thermally mismatched glass tubes with metal housings and glass optical elements with metal holders. In these joints, a high compliance design is used. The silicones, which can be cured between 20-150xc2x0 C. in the presence of moisture, are typically characterized by an extremely low Young""s modulus (E less than 500 psi) and high thermal expansion (xcex1=180-250 10xe2x88x926xc2x0 C.xe2x88x921) A typical example would be DC 577 silicone, which can be used to bond, for example, a metal optical filter holder to a GRIN lens.
Adhesive bonding with subsequent soldering or welding is used to encapsulate a filtering assembly into a three-port package of a DWDM module. A precise alignment achieved during initial assembly of a filter prior to final packaging can be easily decreased due to the adhesive curing process and the high temperature thermal cycles associated with soldering or welding during the final packaging of the components. Such manufacturing processes and resulting components have several problems resulting from stresses on the optical components due to the thermal contraction mismatch between the glass and metal materials, polymerization shrinkage in adhesive bonds, and structural constraints induced by bonding and final soldering during encapsulation. These stresses lead to displacements of optical components during bonding and soldering, resulting in 0.3 to 1 dB or greater increases in the insertion loss.
Such a filter package enclosure, which is typically formed of six to eight concentric protective units, has micron transverse tolerances. Maintaining these tolerances requires precision machining, time-consuming alignment, and soldering with frequent rework. As a result of these limitations, the optical performance specifications are lowered and cost is increased. As an example, soldering typically includes several re-flow cycles. This induces local thermal stresses in the nearby adhesive bonds and leads to the degradation of the polymer adhesive, resulting in repositioning of optical components and a shift in the filter spectral performance. With such design, soldering may also result in the contamination of optical components through direct contact with molten solder and/or flux.
However, for many applications, it is desirable to obtain a high accuracy, thermally compensated optical multiple-port package that can be relatively inexpensive, reliable, and have a low insertion loss. Additionally, a package design should be adequate not only to mechanically protect the fragile optical components, but also to compensate for and minimize the thermally induced shift in spectral performance. Further, it is desirable to obtain a multiple-port package, such as six port packages, with the same qualities since they further reduce costs, reduce size, and also result in reduced insertion loss. Thus, there exists a need for such optical packages and a process for manufacturing such optical packages, which is miniaturized, has a low insertion loss, is inexpensive to manufacture, and which results in a device having reliable, long-term operation.
The present invention provides an improved optical assembly (e.g., optical filter assembly) with a low insertion loss (IL) and provides an assembly of the optical components, such as input ferrules, collimating lenses, and filters, utilizing bonding adhesives in a manner which allows the alignment of the individual components relative to one another with a precision and a manufacturability that makes it possible to produce commercial devices having five, six or more ports. This heretofore had not been achieved. In one aspect, the invention includes an improved input ferrule and filter holder which permits active alignment and bonding through the utilization of UV and thermally curable adhesives and improved thermal curing to greatly reduce relevant internal stresses in the subassembly so formed. For assemblies having multiple pairs of fibers (e.g., five or more port devices), the invention also provides improved fiber ferrule designs, alignment methods, and methods to permit the manufacture of devices that have low IL, operate over a wide temperature range, are reliable, and cost effective.
In one aspect of the invention, improvements to fiber ferrules are provided including capillary designs and tolerances. The invention provides designs for capillaries which resist movement of the optical fibers during adhesive curing, soldering, welding, and environmental thermal changes. One technique uses washers to precisely position optical fibers in a capillary. Yet another aspect of the invention is the selection of optical fibers based on geometric properties such as: outer (cladding) diameter, circularity of the cladding (ovality), and core concentricity. In another aspect, the invention teaches matching the separation distance (SD) between optical fibers and the relationship to angle of incidence (AOI) of the optical filter. Tolerances for the separation distance are provided which make possible the commercial manufacturability of multiple-port devices with five, six or more ports. The optical alignment process becomes more critical and complex as the number of ports increases and therefore the invention provides methods for handling this more complex alignment. A method of selecting an output collimating assembly is also provided.
Methods embodying the present invention include the steps of actively aligning a filter holder and filter to a collimator assembly including a GRIN, aspheric, or other collimating lens mounted thereto, axially separating the filter holder and lens in a movable fixture, placing a UV and thermally curable adhesive on the periphery of the lens, moving the lens into engagement with the filter holder having a filter mounted therein, aligning the collimator assembly with respect to the filter holder while monitoring the input and reflected signals of the optical fibers coupled to the lens for insertion loss less than about 0.2 dB, and applying UV radiation through the filter end of the filter holder to initially cure the aligned subassembly. In an embodiment of the invention, the subassembly is subsequently thermally cured through an accelerated dark cure sequence followed by a final high temperature curing. In another embodiment of the invention, UV radiation is applied to the filter holder/lens interface through one or more apertures formed in the side of the filter holder which overlaps the lens. The UV light source may be dithered such that UV radiation uniformly covers the cylindrical interface between the filter holder and the outer surface of the lens. In yet another embodiment of the invention, the filter and lens are pre-aligned prior to the application of adhesive by monitoring the input and reflected signals of the fibers while adjusting the X-Y positioning for a maximum detected signal.
In a preferred method of manufacturing the invention, subsequent to the UV curing process, the assembly is cured through a stress relaxation cycle at about 40-50xc2x0 C. for two to four hours followed by a thermal curing cycle of about 95-110xc2x0 C. for one to two hours.
In one embodiment of the invention, an input ferrule is employed with an input cone having an axial length greater than about 2.5 mm to reduce S-bending of input fiber, thereby minimizing resultant insertion losses. In another embodiment of the invention, a generally cylindrical filter holder having an annular seat formed in one end for receiving a filter and a lens-receiving aperture at an opposite end having an internal dimension which allows micro-tilting of the filter holder relative to the lens is used to provide an alignment of the filter at an angle of less than about 1xc2x0 to the axis of the lens. The preferred filter holder includes slots or openings in the lateral surface such that UV light enters and cures adhesive between the lens and filter holder. An optical filter assembly of a preferred embodiment of the present invention includes such an improved ferrule and/or a filter holder coupled in alignment with one another in a suitable housing.
The methods and apparatus described herein facilitate the manufacture of a multiple-port optical device which results in several advantages. For example, in a six port device having two pair of optical fibers in the input collimating assembly, one filter operates with at least two transmitted light beams and splits the beams into at least two reflected and two transmitted beams, thereby reducing by half the number of optical filters, collimating lenses and enclosure units. Thus, for example, the same six-port filtering package can be used in the multiplexing and de-multiplexing operations of a DWDM module incorporating concatenated six-port packages. A typical DWDM module includes from two to eight six-port packages. In this case, the number of filter chips, collimating lenses, and fiber ferrules will be reduced by one-half compared to using three port packages.
The manufacturing method and optical element assembly of the present invention, therefore, provides an improved performance optical assembly utilizing a unique input ferrule, filter holder, and an assembly method for providing a low cost, highly reliable, and improved performance optical element assembly, such as a three-port collimating filter or six-port collimating filter assemblies, and using these assemblies in DWDM modules which can be used in an optical communications system.
The devices of the instant invention are applicable for both single mode optical fibers that are applicable to DWDM operations and for polarization maintaining fibers that can be used in the crystal based isolators, circulators, polarization splitters, and the like.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.